My first choice to counter DP would be to use a squad of Grey Knight Terminators or Paladins. I also suspect that an IG army loaded down with max. lascannons could also ruin his day. It also depends on how smartly the daemon player protects and screens his DP with lesser units like daemonettes.
Another thing that has lessened the DP effectiveness (IMO) is that in 7th ed. he can no longer go from Swooping to Glide mode (or is that vice-versa?) and still assault in the same turn. That is huge and leaves him vulnerable to fire for a turn (just as if he had used Deepstrike). Of course, in my last game I also used some psyker ability to increase his invuln. save from a 5+ to a 4+. That was fun and new to me!